Abstract
Cellular memory is a critical ability that allows microorganisms to adapt to potentially detrimental environmental fluctuations. In the unicellular eukaryote Saccharomyces cerevisiae, cellular memory can take the form of faster or slower responses within the cell population to repeated stresses. Using microfluidics and fluorescence time-lapse microscopy, we studied how yeast responds to short, pulsed hyperosmotic stresses at the single-cell level by analyzing the dynamic behavior of the stress-responsive STL1 promoter (pSTL1) fused to a fluorescent reporter. We established that pSTL1 exhibits variable successive activation patterns following two repeated short stresses. Despite this variability, most cells exhibited a memory of the first stress as decreased pSTL1 activity in response to the second stress. Notably, we showed that genomic location is important for the memory effect, since displacement of the promoter to a pericentromeric chromatin domain decreased the transcriptional strength of pSTL1 and led to a loss of memory. This study provides a quantitative description of a cellular memory that includes single-cell variability and highlights the contribution of chromatin structure to stress memory.
Highlights
Cellular memory can be defined as a cellular response to transient and repeated stimuli
Individual cells could be tracked during the time course of an experiment, allowing us to quantify the activation of pSTL1 in response to the first and second osmotic stresses due to the short, transient activation of the high osmolarity glycerol (HOG) pathway (Figure 1C)
Daughter cells born between the two stresses were not considered, as they were not exposed to the first stress and may blur the stress response
Summary
Cellular memory can be defined as a cellular response to transient and repeated stimuli. Active genetic responses that allow cells to survive a single stimulus are termed cellular adaptation. The response of budding yeast to osmotic changes has proven a useful tool to study the emergence of adaptation and cellular memories in this organism [20,21]. Despite the existence of this pronounced dynamic variability, most cells exhibited the same behavior, namely, the response to the second stress was reduced in amplitude We termed this specific behavior the memory effect. This study suggests that the specific location of pSTL1 at the subtelomere is necessary for the optimal level of transcription required to go beyond simple stochastic behavior and to enable the emergence of memory in response to short osmotic stresses
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